The uses of stable isotopes in biomedicine have been explored. Synthesis and behavior of deuterium and carbon-13 labelled alkanes, deuterium labelled serotonin and its congeners and deuterium labelled desipramine and imipramine have been recorded. BIBLIOGRAPHIC REFERENCES: Shaw, G.J., Wright, G.J., and Milne, G.W.A.: Mass Spectra of Some Specifically Deuterated Tryptamines. Biomed. Mass Spectrom., 4, ppp, (1977). Shaw, G.J., and Markey, S.P.: The Synthesis of N-deuteromethylated Desipramine and Imipramine. Biomed. Mass Spectrom., 4, ppp, (1977).